High frequency part



.Aug. 16, 1938. H. VOGT 2,127,241

HIGH FBEQUENCY PART File d Aug. 5, 1953 i lllllilllllll A i MW mm Patented Aug. 16, 1938 UNITED STATES PATENT OFFICE HIGH FREQUENCY PART Application August 3, 1933, Serial No. 683,507 In Germany August 3, 1932 9 Claims.

The present invention relates to band pass filters for broadcast receivers for frequencies above k. c. It is the object of the invention to produce band pass filters of simple construction and compact size in screened condition, but possessing at the same time a resonance curve of very steep flanks and having high amplification.

A band pass filter according to the present invention comprises two tuned circuits, each including a tuning coil having a magnetic core of low loss at frequencies above 150 k. 0., combined with a multi-layer concentrated low capacity winding and an external condenser across each coil, all of these elements being enclosed in a common screening can. The two. tuned circuits are more or less coupled with one another by the mutual inductance between the tuning coils. On the other hand, however, due to the small stray field of the iron cored coils, the two coils can be arranged close to one another without producing too tight a coupling.

Band pass filters with inductance coils accommodated in a common screening can are known from the prior art. Such filters, however, so far were not equipped with magnetic cores of low loss at frequencies above 150 k. c., and either had to be made very large in size or did not produce the steep flanks of the resonance curve and the high amplification, which is desirable in highly efficient broadcast receivers. All of these favorable properties are attained by the combination of elements according to the present invention. Now, it could be assumed that this drawback would be more important still in case of magnet core coils, due to the much more intensive fields of iron core coils. I have found out, however, that, if particular coil shapes with a substantially closed magnetic core and with a particular core type made according to the method disclosed in my copending application Serial No. 721,758 filed April 21, 1934, (Patent 2,064,773), or in my Patent Nos. 2,011,697 and 2,011,698, are used, in particular toroid coils and shell coils such as are shown and described in my copending applications Serial Nos. 674,847 and 674,848, both filed June 8, 1933 (Patents 2,064,771 and 2,064,772), such coils on the contrary can be arranged very close to each other and also the common screening cover can be arranged very tightly around the coil without increasing the damping. This may be explained by the fact that, due tothe closed iron path and the higher pe m ability of the magnet material, the field in the core proper is very intensive, but the stray field in air is extremely weak.

According to the invention band pass filters may be built, the coils of which are arranged very close to each other without obtaining too tight inductive coupling. The band pass coils for several wave ranges may be fitted in the same screening can without effecting undesirable coupling. The remaining coupling elements, for example condensers and resistances, may be arranged in the same can. Moreover the band pass filters or coil sets for several wave ranges may have the wave switch built in.

Some embodiments of the invention will be described in detail in accordance with Figs. 1-4 of the accompanying drawing but there may of course be found many other arrangements under the scope of this invention.

In the drawing Figs. 1 and 2 show an intermediate frequency band pass filter according to the invention, in elevation and in section. The appertaining condensers are built in, and Figs. 3 and 4 show a band pass filter for several wave ranges in elevation and in top plan view respectively.

Referring to Figs. 1 and 2, two tuned oscillatory circuits are arranged in the metal casing 30, said circuits having the conventional arrangement (not shown here) and consisting of the transformers 3| and 33 and the appertaining condensers 32 and 34, the transformers possess a core of magnetic material having low losses. Therefore they have a considerable inductance so that a higher resonance resistance and thus a better amplification is effected. Due to the particular permeability of the magnet core as described in my aforementioned applications (about 10 to 20) the damping of the circuits is extremely small and resonance curves of very steep flanks can be obtained. The variable capacities may consist of squeezing condensers, which can be adjusted by the screws 39 and 49. The coupling between the two transformers, which as mentioned, are used mainly for superheterodyne receivers, but may be used for other purposes as well, is a more or less inductive one by the small stray field of the transformers.

With respect to the coupling between the two tuned circuits aforementioned, the following is stated. With this coupling, fundamentally the same conditions exist as prevail in band pass filters with air cores. The band pass filters, Figs. 1 and 2, concerning the coupling between the transformers 3| and 32, are distinguished from the prior art air core transformers only by the feature that the coils are wound on iron cores of a specific type, previously referred to, instead of air cores, and that these iron cores are closed or substantially closed, so that the magnetic lines of force of the main field run throughout within the core material. Nevertheless, as is well known, a very small stray field still exists, and this stray field is utilized for coupling the two transformers. This coupling, as all couplings between two oscillatory circuits tuned to the same frequency, pro duces a two-wave effect as in all band pass filters, so that the well-known band pass filter resonance curve with two humps is produced. Ac cording to the nature of the band pass filter, these two humps of the curve are the closer to gether the looser the coupling between the present transformers 3i and 32, and are the further apart the closer the coupling. The difference over the prior art air core transformers is, however, that in the present case the coupling between the iron core coils, due to the extremely small stray field, is so loose, notwithstanding the very close proximity of the two transformers, that the band pass resonance double curve becomes very narrow, and thus the frequency band is very sharply defined on both sides in this compact form of filter arrangement. It may be stated already here that the same conditions prevail in the band pass filter systems shown in Figs. 3 and 4 to be described later.

The effect of these stray fields and so the coupling can be regulated partly by the distance be tween the cores, partly by an electric screen 35 movably arranged between the two transformers. The base plate of the variable condensers is so arranged that it presses against the inner wall of the metal casing, thus specially fixing the transformers. After the oscillatory circuits have been matched and the most favourable coupling and the correct band width has been adjusted, the whole arrangement may be: filled up with paraflin or a similar material of proper insulating qualities. The connections are led out by means of terminals which are pressed into thin insulating strips and are led out through holes at the circumference of the casing. The sheet metal casing also has openings to vary the tuning of the circuits. After the casing 31 has been filled up, it is closed by a bottom plate 36, which has an attaching border 31 and is connected by riveting to the casing which supports the whole arrangement. The metal casing 30, the so-called screening can, has no influence upon the characteristic of the band pass filter, and merely serves for shielding the enclosed filter system from stray fields emanating from other elements of the system in which the filter is installed and for screening these other circuit elements from the stray field of the filter.

For building up the band pass filter according to Figs. 3 and 4 the inductance elements developed by the applicant have been used, consisting of shell coils or semi-shell coils respectively which are arranged in transparent rectangular insulating casings of a material of low losses (for instance the press masses which are on the market under the name of Trolitul and consist mainly of acetyl cellulose) and comprise a device for accurately adjusting the inductance of each individual coil, when fitted (for instance by air gap adjustment as described in my aforementioned application Serial No. 674,848)

Figs. 3 and 4 show the coils arranged in transparent insulating covers. The long wave coils 43 and 44 are arranged in the lower casings 41 and 42, the short wave coils 53 and 54 in the higher casings 45 and 46. The corresponding coils accordingly are arranged diagonally to obtain the right coupling. By varying the distance or putting in a screening plate the coupling and therefore the band width can be varied. The long wave casings and the short wave casings are of different height, but they have all the same base, so that a uniform column of coils is resulting. Each coil can be very accurately matched to a certain inductance value by means of a little adjusting screw 41. This adjusting screw controls the size of a small air gap in the otherwise closed magnetic circuit in a manner shown and described more specifically in my aforementioned copending application Serial No. 674,848. In the present case, it will be noted in Fig. 4, a portion 53 of the main core portion 53 of inductance element 53 is shown slightly removed from portion 53 and by an adjustment by screw 41 above referred to the size of the gap and thus the inductance value of the element can be very finely controlled. This offers considerable advantages in so far as not only the coupling between the two corresponding coils has to be adjusted to a predetermined value but also the inductance of each individual coil, which is influenced by the coupling, must be matched to a predetermined value. Now, in the present case the right cou pling may first be effected by adjusting the dis- 1.:

tance and hereafter each coil is readjusted once more to the correct value of inductance.

In'some of the annexed claims I have characterized the magnetic core of the filter transformer as being of the substantially closed magnetic field type. By this designation I mean a core offering a closed or substantially closed iron core path for the magnetic lines of force. Such cores are used in the band pass filter shown in Figs. 3 and 4, and the construction of one of these cores is clearly shown in Fig. 4 at 55, 5B. This core, composed of two component parts 55 and 56, guides the magnetic flux produced by coil 43. The air gaps 51, present between the two core parts and which can be adjusted by the adjusting screw 58 in order to adjust the inductance of the transformer, as described hereinabove with reference to Fig. 3, are nevertheless so extremely small, as compared with the iron path length, that the core, so far as its effect as a conducting medium for the magnetic flux is concerned, still constitutes for all practical purposes at least a substantially closed core. For instance, in Fig. 2, the joints between the core portions are shown at 3 and 33, and there a practically entirely closed core is shown.

The projecting cam switch 48 is arranged horizontally below the coils. The other coupling elements, that is condensers 49 and resistances 50 are fitted in the aggregate. The screening cover 5| can be removed and is attached only by friction at the edges of the lower frame 52, so that the interior parts are always accessible. The coils may have tappings to adapt them to the conditions of the respective receiver, such as length of the aerial etc. moreover windings for reaction coupling.

According to the invention the band pass filters of radio receivers may be simplified and reduced in size, without detrimentally affecting the losses of the oscillatory circuits.

I claim:--

1. An intermediate frequency band pass filter for broadcast receivers, comprising two self-inductance elements having each a multi-layer concentrated winding and a magnetic core of the substantially closed magnetic field type and of a permeability of about 10-20, and two condensers respectively associated therewith to form two circuits tuned to the same frequency and coupled sufficiently loose to produce a band pass filter, all enclosed in a common screening can.

2. A band pass filter for broadcast receivers, comprising two self-inductance elements having each a multi-layer concentrated winding and a magnetic core of the substantially closed magnetic field type and of low losses at radio frequency, each of said cores having an adjusting screw for opening the closed magnetic circuit to a suitable extent for adjusting the inductance value, and two condensers respectively associated therewith to form two circuits tuned to the same frequency and coupled sufiiciently loose to produce a band pass filter, all of said elements being enclosed in a common screening can.

3. An intermediate frequency band pass filter for broadcast receivers, comprising in combination two self-inductance elements having each a multi-layer concentrated winding and a magnetic core of low losses at radio frequency, said cores being of the substantially closed magnetic field type, the magnetic coupling of said inductance elements being adjustable by moving said inductance elements with respect to each other, and two condensers respectively associated therewith to form two circuits tuned to the same frequency and coupled by the adjustment of said inductance elements sufiiciently loose to produce a band pass filter of sharply defined sides, all of said elements being enclosed in a common screening can.

4. A band pass filter for frequencies above 150 k. 0., comprising two tuned circuits, each including a tuning coil having a magnetic core of low loss at high frequencies above 150 k. c., and a multi-layer concentrated low capacity winding and an external condenser across each coil, all of said elements being enclosed in a common screening can, said two tuned circuits being at least partially coupled with one another by the mutual inductance between said coils.

5. A band pass filter for frequencies above 150 k. c., comprising two tuned circuits, each including a tuning coil having a magnetic core of a permeability of the order of 10, but not exceeding 20, and of low loss at high frequencies above 150 k. c. and a multi-layer concentrated low capacity winding and an external condenser across each coil, all of said elements being enclosed in a common screening can, said two tuned circuits being at least partially coupled with one another by the mutual inductance between said coils.

6. A band pass filter for frequencies above 150 k. 0., comprising two tuned circuits, each including a tuning coil having a magnetic core of low loss at high frequencies above 150 k. c. and a sectionalized multi-layer concentrated low capacity winding and an external condenser across each coil, all of said elements being enclosed in a common screening can, said two tuned circuits being at least partially coupled with one another by the mutual inductance between said coils.

7. A band pass filter for frequencies above 150 k. 0., comprising two tuned circuits, each including a tuning coil having a magnetic core of low loss at high frequencies above 150 k. c. and a multi-layer concentrated low capacity winding and an external condenser across each coil, all of said elements being enclosed in a common screening can, said two tuned circuits being at least partially coupled with one another by the mutual inductance between said coils, and means disposed between said coils for adjusting their mutual inductance.

8. A band pass filter for frequencies above 150 k. c., comprising two tuned circuits, each including a tuning coil having a magnetic core of low loss at high frequencies above 150 k. c. and a multi-layer concentrated low capacity winding and an adjustable external condenser across each coil, all of said elements being enclosed in a common screening can, said two tuned circuits being at least partially coupled with one another by the mutual inductance between said coils, and means disposed between said coils for adjusting their mutual inductance.

9. A band pass filter for frequencies above 150 k. 0., comprising two tuned circuits, each including a tuning coil having a magnetic core of low loss at high frequencies above 150 k. c. and a multi-layer concentrated low capacity winding and an external condenser across each coil, all of said elements being enclosed in a common screening can, said two tuned circuits being at least partially coupled with one another by the mutual effects of at least one of their corresponding circuit constants.

HANS VOGT. 

